The closed beta option is really good starting out, to me Gmail nailed that one with it's invite mechanism. Dropbox is sort of the next step along the way, they

The closed beta option is really good starting out, to me Gmail nailed that one with it's invite mechanism. Dropbox is sort of the next step along the way, they're wide open for signup, but they want more users. Giving away space for invites is a great way to encourage sharing. It's bringing people closer to the paid product experience without requiring spending and getting new users at the same time. Linode does a great job of rewarding existing customers for bringing in new, paying customers.